These data were collected by the SHOALS (Scanning Hydrographic Operational Airborne
Lidar Survey) system which consists of an airborne laser transmitter/receiver capable
of measuring 400 soundings per second. The system operates from a deHavilland DHC-6
Twin Otter flying at altitudes between 200 and 400 meters with a ground speed of about
100 knots. The SHOALS system also includes a ground-based data processing system for
calculating acurate horizontal position and water depth. Lidar is an acronym for LIght
Detection And Ranging. The system operates by emitting a pulse of light that travels
from an airborne platform to the water surface where a small portion of the laser
energy is backscattered to the airborne receiver. The remaining energy at the water's
surface propogates through the water column and reflects off the sea bottom and back
to the airborne detector. The time difference between the surface return and the bottom
return corresponds to water depth. The maximum depth the system is able to sense is
related to the complex interaction of radiance of bottom material, incident sun angle
and intensity, and the type and quantity of organics or sediments in the water column.
As a rule-of-thumb, the SHOALS system should be capable of sensing bottom to depths
equal to two or three times the Secchi depth.